Modem neuroscience is only beginning to understand how the activity of forebrain circuits is modulated by neurochemicals, including steroids such as estrogen. Appreciation of the neuroprotective effects of steroids has led to ongoing clinical trials for steroid treatment of human neurocircuit pathologies. Current clinical treatments deliver steroids peripherally, and therapeutic steroids consequently reach the brain slowly in a uniform, global manner. However, only a subset of steroid treatments appears to be effective at reducing neurological disease progression. The most recent, emergent research on the actions of steroids within the brain emphasizes both local and acute effects on neurons and neurocircuits. In order to optimize our use of steroids as neuro-therapeutic agents, we must therefore consider the fine temporal and spatial scale of steroid regulation within neurocircuits. With the advancement of in vivo neurosteroid microdialysis, I have established that forebrain steroid levels can be experimentally monitored and manipulated in discrete neurocircuits. In this K99/R00 career development award, I will determine how local steroid actions contribute to forebrain circuit activity and function. During the Mentorship Phase, I will acquire skills in electrophysiological, functional, and developmental approaches to forebrain circuits, and combine them with recently- optimized in vivo neurosteroid microdialysis techniques. I have assembled a highly-qualified team of mentors and collaborators with a comprehensive set of skills to facilitate my training and maturation into an independent neuroscientist. During the Independent Phase, I will direct my laboratory to apply these innovative tools to a broad- scale, integrative study of how forebrain circuits are modulated by local, brain-derived steroids. Results of the training and research outlined in this K99/R00 Award will provide an integrated understanding of neural circuit regulation, and could have wide-ranging implications for the future development of novel therapeutic approaches to neurological diseases.